1. Field of Invention
The present invention relates to a method for producing an Fexe2x80x94Ni alloy with improved punching property, which is suited for the electrode material of an electron-gun.
2. Description of Related Art
FIG. 1 is a cross-sectional view of the known shadow-mask type color cathode-ray tube. A fluorescent coating 2 is applied on the panel 1 and emits three primary colors, i.e., red, green and blue. An electron-gun 4 is provided in the neck of the cathode-ray tube and emits electron beams 3. The electron beams 3 are deflected and scanned by the deflecting yoke 5. Reference numerals 6 and 7 denote the shadow mask and the magnetic shield, respectively. All of these parts 1 through 7 are known.
FIGS. 2(A) and (B) are the illustrative view and cross-sectional view of an electrode (grid electrode) of the electron gun 4, respectively. The minute apertures 10a, 10b and 10c are formed by coining and punching a sheet and let the respective electron beam pass therethrough for generating the colors red (10a), green (10b) and blue (10c). For the parts of an electron-gun used in a color cathode-ray tube, a non-magnetic stainless steel sheet having a thickness of from 0.05 to 0.5 mm has conventionally been used, since the non-magnetic property is important for the electrodes 10 which accelerate the electrons emitted from the cathode of the electron gun. Such a sheet is intermediately coined and then blank-worked as above or is directly blanked without coining.
Recently, rather than the non-magnetic property, a low thermal expansion property is considered to be more important, because the color cathode-ray tube of a computer-display or the like has become highly precise and its performance is enhanced. In this case, picture performance (purity of color) is influenced by subtle dimensional changes in the electrode 10 due to its thermal expansion.
Since an Fexe2x80x94Ni alloy, particularly Fexe2x80x9442% Ni alloy (the so-called 42 Alloy), has a low thermal expansion property, its use as the electrode material has now been undertaken. However, when the conventional 42 Alloy is punched by means of a punch to form the minute apertures 10a, 10b and 10c, burrs are detrimentally formed at the front end 10e (FIG. 2(B)), where the punching tailings are separated from the sheet. The punching burrs exert a detrimental effect upon the dimensional accuracy of the electron-gun part which is required to be highly precise. Minimizing of the burrs of an electrode-gun part will be more and more highly demanded along with precision of the color cathode-ray tube.
There are, heretofore, proposals in Japanese Unexamined Patent Publications Nos. 6-122,945, 6-184,703, 7-3,400 and 7-34,199 to improve the press-punching formability of an Fexe2x80x94Ni alloy. In Japanese Unexamined Patent Publication No. 6-184,703, the S content of an Fexe2x80x94Ni alloy is limited to a range of 0.002 to 0.05%, and the sulfur or sulfur compound is dispersed along the grain boundaries or within the crystal grains. However, the burrs cannot be satisfactorily prevented only by means of adding a specified amount of sulfur; hence, efforts to improve the machinability, when the alloy is subjected to the extremely precise press forming, have been carried out in recent years.
Next, the proposals made in Japanese Unexamined Patent Publications Nos. 6-122,945, 7-3,400and 7-34,199, reside in that such strengthening elements as Ti, Nb, V, Ta, W, Zr and the like are added to enhance the hardness and hence to induce a degree of brittleness thereby suppressing the burrs. The life of a metal die is, however, shortened due to the increased hardness of the Fexe2x80x94Ni alloy.
It is an object of the present invention to eliminate the above described drawbacks and to provide a method for producing an Fexe2x80x94Ni alloy, the press-punching formability of which required for the press-formed part of an electron gun is improved.
The present inventors considered various ways to find a relationship between the press-punching formability and inclusions and a relationship between the distribution of inclusions and the production conditions.
As a result, it was discovered that the press-punching formability is improved by means of limiting the Ti, Mg, Ce, Ca and S contents in a specified range and hot-working in an appropriate range of temperature which is dependent upon the above contents. More specifically, an appropriate amount of Ti, Mg, Ce and Ca sulfides formed in the Fexe2x80x94Ni alloy can promote propagation of cracks generated during the press-punching, thereby improving the press-punching formability. The heating temperature at the hot-working exerts great influence upon the amount and distribution of the sulfides which are important for improving the press-punching formability, particularly for suppressing the burrs.
Furthermore, Ti, Mg, Ce and Ca are mainly combined with sulfur to improve the press-punching formability but are not dissolved in the matrix of the alloy appreciably. These elements do not, therefore, result in the hardness increase and hence shortening of the life of a metal die.
The method according to the present invention is based on the above discoveries and comprises the steps of:
melting an Fexe2x80x94Ni alloy, which contains, by weight percentage, from 30 to 55% of Ni, not more than 0.8% of Mn, from 0.001 to 0.050% of S, from 0.005 to less than 0.5% of at least one element selected from the group consisting of Ti, Mg, Ce and Ca, the balance being Fe and unavoidable impurities, and in which the product of the total content of said Ti, Mg, Ce and Ca and the content of S is limited in a range of from 0.00005 to 0.0100;
casting the melted alloy;
heating the alloy prior to hot-working at a temperature (T) under the following equation,
1050xe2x89xa6T(xc2x0C.)xe2x89xa6{19500/[8.5xe2x88x92log[%X][%S]]}xe2x88x92350,
in which the S content is denoted by [%S], the content of the total amount of Ti, Mg, Ce and Ca is denoted by [%X],
hot-working the heated Fexe2x80x94Ni alloy;
cold-rolling; and,
annealing.